1. Field of the Invention
The present invention relates to a non-interruptive protection switching device and a network system. More particularly, the present invention relates to a non-interruptive protection switching device which instantaneously switches data traffic from working channels to protection channels, as well as to a network system which employs east and west transmission subsystems with non-interruptive protection switching capabilities.
2. Description of the Related Art
Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) transmission systems are widely used as the core technology in today""s telecommunication infrastructures. In general, SDH/SONET transmission systems are configured with a multiple redundant architecture to provide higher reliability and availability. When a problem occurs with a working channel, the system will immediately switch the traffic signals from the failed channel to a protection channel, thereby preventing the communication from being disrupted. This protection switching function is referred to as xe2x80x9cnon-interruptive switchover.xe2x80x9d
Since the working and protection channels have different propagation delay characteristics, there is a need for the phase synchronization of signal frames at the receiving end to make non-interruptive switchover operations possible. FIG. 14 briefly explains how a conventional transmission unit performs this phase adjustment. The transmission unit receives a stream of frames F1 through the working channel, as well as F2 through the protection channel. Note that the protection-channel frames F2 arrive at the unit with a delay time of t1 with respect to the working-channel frames F1. This phase difference between F1 and F2 must be eliminated when the transmission unit performs a protection switching operation. As FIG. 14 shows, the non-interruptive protection switching facility of the unit adjusts their phases and sends out the resultant in-phase frame signals F1a and F2a through its working and protection facilities, with a delay time of t2 after the reception of F2. The delay time t2 has to be determined considering various parameters including: the time required for line switching after failure detection, and an extra delay time resulting from additional equipment placed on the transmission line. In this way, the conventional transmission unit switches from one transmission subsystem to another without disrupting the traffic when a failure occurs.
The conventional non-interruptive protection switching device, however, is not robust enough to cope with possible variations in the phase of incoming frames, which may occur suddenly for some reason. In such a problem situation, its internal control operation to read frame data would be adversely affected by the phase variation, resulting in main signal errors.
Another problem with the conventional non-interruptive protection switching devices is that, when the transmission clock was lost, they would do nothing but reporting that failure to the operator. That is, the conventional devices lack the ability to properly handle such clock loss faults not to disrupt its internal control.
Still another problem with the conventional devices is their inability to handle concatenated payloads carrying large-capacity signals. That is, they are not flexible enough to control write and read operations in various multiplex modes using concatenation techniques. To realize such different multiplex modes in a conventional device, more complex circuitry would be necessary.
Taking the above into consideration, an object of the present invention is to provide a non-interruptive protection switching device which performs protection switching in a more efficient and robust way to provide a higher quality communication service.
It is another object of the present invention to provide a network system which performs protection switching in a more efficient and robust way to provide a higher quality communication service.
To accomplish the first object, according to the present invention, there is provided a non-interruptive protection switching device which switches data traffic from working channels to protection channels. This non-interruptive protection switching device comprises a working-channel transmission controller, a protection-channel transmission controller, and a read controller.
The working-channel transmission controller comprises the following elements: a working-channel signal storage unit which stores data signals received through a working channel, where the data signals contain working-channel main signals for a multiframe interval; a working-channel multiframe synchronization controller which monitors multiframe patterns in the working-channel main signals to detect a multiframe synchronization timing thereof, provides a working-channel multiframe sync detection pulse signal when the working-channel main signals are in a normal condition, and stops the provision of the working-channel multiframe sync detection pulse signal when the working-channel main signals fall into a faulty condition; and a working-channel write controller which controls write access to the working-channel signal storage unit by providing a working-channel write pulse signal therefor, and produces working-channel write phase data by identifying a phase difference of the detected multiframe synchronization timing with respect to a reference phase signal. Here, the working-channel write pulse signal is produced from the multiframe sync detection pulse signal when the working-channel multiframe sync detection pulse signal is available, or from a free-running timebase when the working-channel multiframe sync detection pulse signal is stopped.
The protection-channel transmission controller comprises the following elements: a protection-channel signal storage unit which stores data signals received through a protection channel, where the data signals contain protection-channel main signals for a multiframe interval; a protection-channel multiframe synchronization controller which monitors multiframe patterns in the protection-channel main signals to detect a multiframe synchronization timing thereof, provides a protection-channel multiframe sync detection pulse signal when the protection-channel main signals are in a normal condition, and stops the provision of the protection-channel multiframe sync detection pulse signal when the protection-channel main signals fall into a faulty condition; and a protection-channel write controller which controls write access to the protection-channel signal storage unit by providing a protection-channel write pulse signal therefor, and produces protection-channel write phase data by identifying a phase difference of the detected multiframe synchronization timing with respect to a reference phase signal. Here, the protection-channel write pulse signal is produced from the multiframe sync detection pulse signal when the protection-channel multiframe sync detection pulse signal is available, or from a free-running timebase when the protection-channel multiframe sync detection pulse signal is stopped.
The read controller produces a read pulse signal from the working-channel write phase data and the protection-channel write phase data, and based on the produced read pulse signal, reads out the stored data signals simultaneously from the working-channel signal storage unit and the protection-channel signal storage unit for use in downstream transmission.
Further, to accomplish the second object, according to the present invention, there is provided a network system which employs east and west transmission subsystems with non-interruptive protection switching capabilities. This network system comprises a plurality of transmission units, a transmission medium which interconnects the plurality of transmission units in ring form; and a plurality of non-interruptive protection switching devices. The non-interruptive protection switching device is disposed in each of the transmission units and comprises an east-channel transmission controller, a west-channel transmission controller, and a read controller.
The east-channel transmission controller comprises the following elements: an east-channel signal storage unit which stores data signals received through an east channel, where the data signals contain east-channel main signals for a multiframe interval; an east-channel multiframe synchronization controller which monitors multiframe patterns in the east-channel main signals to detect a multiframe synchronization timing thereof, provides an east-channel multiframe sync detection pulse signal when the east-channel main signals are in a normal condition, and stops the provision of the east-channel multiframe sync detection pulse signal when the east-channel main signals fall into a faulty condition; and an east-channel write controller which controls write access to the east-channel signal storage unit by providing an east-channel write pulse signal therefor, and produces east-channel write phase data by identifying a phase difference of the detected multiframe synchronization timing with respect to a reference phase signal. Here, the east-channel write pulse signal is produced from the multiframe sync detection pulse signal when the east-channel multiframe sync detection pulse signal is available, or from a free-running timebase when the east-channel multiframe sync detection pulse signal is stopped.
The west-channel transmission controller, on the other hand, comprises the following elements: a west-channel signal storage unit which stores data signals received through a west channel, where the data signals contain west-channel main signals for a multiframe interval; a west-channel multiframe synchronization controller which monitors multiframe patterns in the west-channel main signals to detect a multiframe synchronization timing thereof, provides a west-channel multiframe sync detection pulse signal when the west-channel main signals are in a normal condition, and stops the provision of the west-channel multiframe sync detection pulse signal when the west-channel main signals fall into a faulty condition; and a west-channel write controller which controls write access to the west-channel signal storage unit by providing a west-channel write pulse signal therefor, and produces west-channel write phase data by identifying a phase difference of the detected multiframe synchronization timing with respect to a reference phase signal. Here, the west-channel write pulse signal is produced from the multiframe sync detection pulse signal when the west-channel multiframe sync detection pulse signal is available, or from a free-running timebase when the west-channel multiframe sync detection pulse signal is stopped.
The read controller produces a read pulse signal from the east-channel write phase data and the west-channel write phase data, and based on the produced read pulse signal, reads out the stored data signals simultaneously from the east-channel signal storage unit and the west-channel signal storage unit for use in downstream transmission.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.